Du Pont Co. reported sharply higher earnings in 1978 yesterday and announced plans to boost its dividend and split its stock three for one.

The announcements sent Du Pont shares soaring on the New York Stock Exchange, where the stock colsed at 143 1/3, up 11 1/4.

For the fourth quarter, Du Pont earnings rose 84 persent to $219 million ($4.50 a share) from $119 million ($2.42) in the same period of 1977. Sales rose 15 percent to $2.68 billion from $2.33 billion.

Du Pont said its earnings for the year climbed 44 percent to $786 million ($16.15) from $545 million ($11.06).

Chairman Irving S. Shapiro said Du Pont's chemicals, plastics and specialty products businesses in 1978 posted solid gains for the third straight year.

Sales for 1978 increased 12 percent to $10.58 billion from $9.43 billion, reflecting a 7 percent increase in physical volume and a 5 percent increase in selling prices, Shapiro said.

"Despite the widespread concerns about the U.S. economy which have been voiced by some in recent months, we have not seen evidence of a downturn so far," Shapiro said. "While we expect some slowing of the U.S. economy in 1979, our business outside the U.S. is expected to show continued growth."

Amendments to the certificted of incorporation effecting the split will be submitted to the company's common stock holders at a meeting April 16. If approved, the stock split will become effective May 30.

Directors at the board meeting stated their intention to raise the 1979 first-quarter dividend to $1.50 a share from $1.25 paid in each of the first three quarters of 1978. After the stock split, the $1.50 interim dividend rate will translate to 50 cents a share.

The year-end 197, dividend of $3.50 a share included the $1.25 interim dividend plus an added $2.25 a share.

International Business Machines Corp., the computer giant, reported yesterday that net earnings for 1978 rose 16.3 percent to $21.29 a share from $18.30 a share in 1977.

Last year, IBM had net income of $3.11 billion, up 14.4 percent from $2.72 billion in 1977. Revenues climbed 16.2 percent to $21.08 billion from $18.13 billion the year before.

In the 1978 fourth quarter, the company earned $1.01 billion ($6.95 a share), up from $797 million ($5.38) in the year-earlier period. Revenues totaled $6.44 billion against $5.-4 billion in the final 1977 quarter.

IBM's results for 1978 included $113 million in currency exchange gains, up from $28 million in 1977. But fourthquarter exchange gains amounted to $14 million, down from $64 million in the closing 1977 quarter.

Purchases of data processing equipment set records for both the fourth quarter and full year 1978.

Incoming orders were strong in 1977 and, even with a high shipment rate, IBM's backlog at year-end exceeded the previuous year's level.

Bendix Corp., a major supplier to the auto industry, reported a 32 percent increase in profits during the October quarter amid surging sales for its auto components.

Earnings for the quarter, which is the first of Bendix's fiscal year, were $37.6 million ($1.64 a share) against $28.5 million ($1.28) for the same quarter the year before. Revenues for the period rose 8.1 percent to $916.1 million from $847.5 million.

The quarterly performance was the best ever for Bendix. But Chairman William M. Agee said in a statement, "We do not expect to match the first quarter's rate of growth for the rest of the year."

Bendix also reported that it had increased its investment in Asarco, a major copper company, to 20 percent of Asacrco's stock.

NCR Corp a producer of business and electronic ewuipment, recorded higher sales and earnings inthe final 1978 quarter and the full year.

Chairman William S. Anderson siad orders for equipment were at a record level. In light of this and plans to introduce new products this year, he said NCR anticipates "continued growth" in its product line and all major markets.

For the quarter, the company earned $89.99 million ($3.34 a share) against $59.72 million ($2.20) a year earlier. For the year, it earned $318 million ($11.85) against $142.47 million ($5.31) a year earlier. Included in the latest year's net was an extraordinary credit of $110.49 million.

Revenues for the quarter totaled $852.97 million against $717.96 million in the 1977 period. For the year, revenues totaled $2.6 billion against $2.31 billion the previous year.