ແກ້ cos(7pi/4)=

$$\cos ( \frac { 7 \pi } { 4 } ) =$$

ປະເມີນ

$$\frac{\sqrt{2}}{2}\approx 0,707106781$$

ຂັ້ນຕອນການແກ້

ໃຊ້ \cos(x+y)=\cos(x)\cos(y)-\sin(y)\sin(x) ບ່ອນທີ່ x=\frac{3\pi }{2} ແລະ y=\frac{\pi }{4} ເພື່ອໃຫ້ໄດ້ຜົນຮັບ.

$$\cos(\frac{3\pi }{2}+\frac{\pi }{4})=\cos(\frac{3\pi }{2})\cos(\frac{\pi }{4})-\sin(\frac{\pi }{4})\sin(\frac{3\pi }{2})$$

ຮັບຄ່າຂອງ \cos(\frac{3\pi }{2}) ຈາກຕາຕະລາງຄ່າຕີໂກນມິຕິ.

$$0\cos(\frac{\pi }{4})-\sin(\frac{\pi }{4})\sin(\frac{3\pi }{2})$$

ຮັບຄ່າຂອງ \cos(\frac{\pi }{4}) ຈາກຕາຕະລາງຄ່າຕີໂກນມິຕິ.

$$0\times \frac{\sqrt{2}}{2}-\sin(\frac{\pi }{4})\sin(\frac{3\pi }{2})$$

ຮັບຄ່າຂອງ \sin(\frac{\pi }{4}) ຈາກຕາຕະລາງຄ່າຕີໂກນມິຕິ.

$$0\times \frac{\sqrt{2}}{2}-\frac{\sqrt{2}}{2}\sin(\frac{3\pi }{2})$$

ຮັບຄ່າຂອງ \sin(\frac{3\pi }{2}) ຈາກຕາຕະລາງຄ່າຕີໂກນມິຕິ.

$$0\times \frac{\sqrt{2}}{2}-\frac{\sqrt{2}}{2}\left(-1\right)$$

ເລີ່ມຄຳນວນ.

$$\frac{\sqrt{2}}{2}$$